Catheters of various types and sizes have been used by physicians extensively. One use of the catheter is in providing regional anesthesia with minimal physiologic alterations. When used at the start of an operation, regional anesthesia minimizes the total dosage of inhalation or intravenous anesthetic drugs required, hastens awakening, and permits early ambulation. When administered at the conclusion of surgery, regional anesthesia produces post-operative analgesia with reduced risk of respiratory depression.
When prolonged analgesia is required, a catheter is inserted into the caudal or lumbar epidural space to provide intermittent or continuous injections of local anesthetics. Continuous lumbar epidural anesthesia is a well-established and accepted technique in adult patients. It differs from caudal epidural anesthesia by the location where the needle is inserted. Caudal epidural anesthesia, however, is notable for its simplicity, safety, and effectiveness and is one of the most frequently used regional anesthetic techniques for operations below the diaphragm in children.
Epidural and spinal anesthesia require the administration of an anesthetic agent into the epidural or subaractmoid spaces respectively of the spine. Epidural anesthesia requires substantially more anesthetic agent than spinal anesthesia and, if the anesthetist inadvertently penetrates the duraarachnoid membrane while endeavoring to administer an anesthetic agent to the epidural space, a dangerous quantity of anesthetic agent can be placed in the subaradmoid space, possibly causing paralysis or even death.
The majority of physicians prefer the use of the midline approach for spinal puncture. Generally, with the midline approach, the spinal injection is made at the center of the patient's back with the needle oriented in a plane parallel to the centerline of the spine. The needle tip is inserted into the back in a straight line toward the midline of the spine (2) between the second (4) and third (6) lumbar vertebrae, a direction generally represented by the arrow (A) shown in FIG. 1. In this technique the epidural needle passes through the supraspinous, inrterspyious and ligamentum flavum structures before entering the epidural space, hisertion of the needle into the epidural space is complicated by the lack of feedback as to the position of the needle tip, coupled with the imperative need to avoid puncturing the dura mater which surrounds the spinal cord, since there is potential for catastrophic trauma to the spinal cord with the epidural needle. Extreme caution must therefore be exercised in the positioning of the needle tip, which must pierce through the tough, resilient, leather-like ligamentum flavum, and then stop immediately within the narrow epidural space, short of puncturing the dura mater.
The needle must be moved through the ligamentum flavum very slowly and in a carefully controlled fashion. At the same time, pressure is applied to the plunger of the attached syringe which is filled either with air or saline solution. The object is to continuously test for loss of resistance to injection, experienced when the needle lumen enters the epidural space after clearing the ligamentum flavum. This loss of resistance is experienced by little if any resistance to injected air or fluid, and a negative aspiration test then indicates that the needle lumen is properly positioned in the epidural space. Special syringes, known as loss of resistance syringes and characterized by very low friction between the plunger and the barrel of the syringe, are used for positioning the needle lumen in the epidural space. Once correct positioning of the needle is achieved, the resistance syringe is separated from the epidural needle and another syringe, loaded with the anesthetic is attached, after which the anesthetic is injected.
It is important to understand the demands placed upon the anesthesiologist's dexterity by this procedure. It is of critical importance that the needle traverse t ligamentum flavum in a carefully measured and controlled manner. Typically, this is achieved by applying resistance to the advancing needle with the anesthesiologist's non-dominant hand (the left-hand if the anesthesiologist is right-handed) while the dominant hand applies pressure to the plunger of the resistance syringe to test for resistance to injection while at the same time slowly advances the needle. Variations of this technique may be adopted according to personal preference, for example the needle may be advanced continuously while pressure on the syringe barrel is also maintained continuously to test for resistance. In the alternative, the needle is advanced in very small increments, e.g., 1 millimeter, testing for resistance to injection after each advance.
The difficulty of correctly positioning the needle lumen in the epidural space has spurred many attempts to develop methods and devices for detecting and indicating correct needle placement. These approaches have generally exploited the low resistance to injection and subatmospheric pressure characteristic of the epidural space. One such technique involves placement of a drop of saline solution on the open hub of an epidural needle. The drop will be "sucked-in" as the needle lumen enters the epidural space where, for reasons not well understood, prevails sub-atmospheric pressure. Other means used for this purpose include capillary attachments with fluid indicators developed by Odom, or inflated balloons by Macintosh, which deflate upon entering the epidural space. It is also known to use spring loading devices to facilitate the loss of resistance phenomena which occur as the epidural needle passes from the dense ligamentum flavum into the lesser resistance of the epidural space.
U.S. Pat. No. 5,024,662, describes an attachment for a resistance syringe for aiding the anesthesiologist in correct placement of the epidural needle. The attachment has an elastomeric band retained to the syringe barrel by a ring which slides onto the syringe barrel against the finger flange of the syringe to anchor the ends of the elastic band to the barrel while a midportion of the band is pulled by the plunger of the syringe. Consequently, the plunger is urged by elastic force into the syringe barrel, but is held back by fluid, air or liquid in the barrel, until the needle lumen enters the epidural space. At that point the contents of the syringe are injected into the epidural space under the force of the stretched band, providing the anesthesiologist with immediate kinesthetic indication of correct needle placement. While this arrangement works well, disposable elastomeric drivers have been developed by this applicant which are of still greater simplicity and very low cost.
U.S. Pat. No. 4,518,383 teaches an instrument for epidural and spinal anesthesia in which an outer hollow Tuohy.TM. needle has a bent pointed tip to locate the epidural space and an inner hollow needle with a pointed tip projecting forwardly of the outer Tuohy.TM. needle in alignment therewith to penetrate the dura with a minimum of cutting of tissue. Likewise, U.S. Pat. No. 4,737,146 discloses another version of epidural catheter in which a rigid epidural needle is inserted into an epidural space and an epidural catheter is introduced through the needle into the epidural space through a lateral opening in the tip of the needle.
U.S. Pat. No. 5,081,990 describes catheters for epidural injection of drugs with electrodes at the distal end for measuring effect of the drugs. This device, however, is not used for determining the placement of the catheter prior to medicant injection and, by placing electrodes into the epidural space and directly contacting the dural tissue, risks electrochemical reaction between the electrode and the dural tissue in the presence of medicament, saline or body fluids.
U.S. Pat. No. 5,423,877 describes a catheter for simultaneous application of electrical stimulation and infusion of analgesic medication to nerve fibers in the spinal cord. The correct placement of the catheter requires constant communicating with the patient to determine the paresthesia in corresponding dermatome and myotome regions. Therefore, the determination of proper catheter location is only possible and limited to a certain group of patients with conscious, communicable, cooperative, calm and oriented state. In addition, such determination is depended on only subjective but not objective evaluation and the results tend to be unreliable and time consuming to obtain.
Additionally, as the metal electrodes are set distally at the end of the catheter tubing, correct placement of the electrode may be detected, but proper delivery of the injected anesthetic may not be reliably established. For instance, the injected anesthetic may leak proximal to the epidural space if there is any damage to the catheter during the insertion procedure.
Therefore, presently the confirmation of catheter placement can be made only after observing the clinical effects from subsequent local anesthesia drug injection. This clinical effect may take up to twenty minutes, depending on the type of local anesthesia injected. What is needed is a simple, rapid and effective method of determining proper placement of the catheter prior to the introduction of anesthesia.